Abstract : The Atlantic cold tongue is a seasonal cooling of the sea surface temperature south of the Equator between the African coasts and around 30°W during the « cold season » (from May to October). The cooling occurs every year but its intensity, duration and spatial extent vary strongly from one year to another. In spite of the very strong coupling between the Atlantic cold tongue and the West African monsoon, the origin of the Atlantic cold tongue variability is not well described. This thesis aims at filling this gap by improving our understanding of the oceanic processes controlling the variability of the Atlantic cold tongue.
This study focuses on « intense » Atlantic cold tongue events, defined by abnormally strong (weak) cooling, preceded by negative (positive) zonal wind anomalies. Thus « canonical » being studied, that are the most frequent and probably similar in terms of mechanisms. This classification is applied to ten reanalyses and allows to select with good confidence, five events in each group.
These events are studied through realistic simulations. The use of on-line heat budget allows to identify the physical processes that control the formation of cold and warm events. Vertical mixing at the base of the mixed layer is the fundamental process controlling the interannual variability of the cold tongue. During cold events, it increases the cooling between March and July, while it remains weak during warm events. During boreal summer, vertical mixing anomalies are balanced by horizontal advection anomalies of opposite sign. So cold (warm) events are weakened (extended) at the end of the season.
This thesis highlights that it is more appropriate to focus on the wind energy flux because it is more directly related to the activation of vertical mixing, rather than on the surface wind stress. The wind energy flux is relevant since it is also shown to play a major role during intense « non-canonical » events. Finally, the modulation of the vertical velocity induced by the wind tends to adjust i) the mixed layer depth, ii) the intensity of the thermocline, and iii) the vertical shear of the zonal current. These are key parameters of vertical mixing and therefore the cooling rate. Thus, vertical velocity plays an indirect role in the establishment and interannual variability of the Atlantic cold tongue.